This invention relates generally to the damping of pulsations in a high-pressure flow line of a liquid pumping system.
A particular application of this invention is in a high-pressure liquid chromatography system in which a carrier liquid containing a quantity of a sample to be analyzed is delivered to a chromatographic column in pulses by the action of a reciprocating pump. In order to provide a continuous flow of liquid to the chromatographic column, it is necessary to insert a pulse damper in the flow line between the reciprocating pump and the chromatographic column.
In one pulse damping technique used in a prior liquid chromatography invention, a block of plastic material (typically, polytetrafluoroethylene) was placed in the flow line between the reciprocating pump and the chromatographic column. The plastic block stored energy by compressing during that portion of the pumping cycle when the pressure in the flowline was increasing, and subsequently released the stored energy by decompressing as the pressure in the flow line was decreasing, whereby pressure pulses in the outflow from the reciprocating pump over a full pumping cycle were damped. A pulse damper of this kind is described in U.S. Pat. No. 4,024,061.
With such pulse damping devices in which a compressible body was disposed directly in the flowline, the damping characteristics of the compressible body could not generally be matched with the pulsing characteristics of the pump. Although different materials have different compressibility characteristics, the choice of materials for the body to be disposed directly in liquid chromatography flow line was limited to those plastics that are chemically inert with respect to the kinds of solvents used in liquid chromatography. Only a few plastic materials, such as polytetrafluoroethylene (Teflon), and polychlorotrifluoroethylene (Kel-F), can meet this requirement. Thus, the damping characteristics of a pulse damper of this type could not generally be selected on the basis of the pulsing characteristics of the pump.
Furthermore, with prior pulse dampers utilizing a compressible body disposed in the high-pressure flowline, the obstruction of the flow line by the compressible body tended to cause non-laminar flow and turbulence that resulted in cavitations. In chromatographic applications, such cavitations tend to interfere with the analysis of the flow line constitutents. In general, for chromatographic applications, a smooth flow path for the flowline liquid through the pulse damper is to be preferred, because discontinuities in the flow path tend to create "dead volumes" within which minute quantities of the flow line liquid could become trapped. Such trapped quantities of flow line liquid are a source of contamination for chromatograhic analysis systems.
Another pulse damping technique used previously involved passing the flowing liquid into a first chamber that is separated from a larger second chamber by a flexible diaphragm. The larger second chamber is filled with a compressible liquid. For a uniform flow rate through the first chamber, the diaphragm maintains a quiescent configuration in separating the first and second chambers. However, when a pulse occurs in the flowline liquid passing through the first chamber, the diaphragm flexes towards the second chamber, thereby temporarily compressing the liquid in the second chamber for the duration of the pulse. The work done by the energy of the pulse in compressing the liquid in the second chamber effectively damps the pulse. At the end of the pulse, the restoring force of the compressed liquid in the second chamber causes the diaphragm to return to its quiescent configuration.